Set your oven to preheat at 350℉. Thoroughly grease and lightly flour a 10-inch bundt pan, then place it aside.
In a large bowl, blend the softened butter and sugar using an electric mixer, mixing until the texture becomes airy and pale. Pause to scrape the bowl’s sides for an even blend.
Crack in the eggs, one at a time, beating slowly after each addition to maintain a consistent batter.
Incorporate the vanilla extract, then continue to mix until fully combined.
Add in both the sour cream and milk, beating to integrate smoothly.
Gradually add the flour and yellow cake mix, stirring until a uniform batter is achieved.
Transfer the batter into your prepared bundt pan and place it in the oven. Bake for approximately 70 minutes.
At the 40-minute mark, inspect the cake. If browning occurs too quickly on the top, loosely tent a sheet of aluminum foil over the pan to shield it from direct heat.
The cake is done when a toothpick inserted into the center emerges clean.
Allow the cake to cool in the pan for 10 minutes before inverting it onto a rack or platter. Slice once cool and serve as desired.